FIG. 7 shows a projector already proposed which comprises three liquid-crystal panels 7, 7a, 7b corresponding to R, G, B, such that the panels are irradiated with beams from a light source 35 to project color images by combining the beams passing through the liquid-crystal panels. In the following description, a beam, opposite ends of which are parallel to the optical axis, will be referred to as parallel rays.
Inside a chassis 3, the liquid-crystal panels 7a, 7b corresponding respectively to B and R are arranged as opposed to each other on opposite sides of the optical axis of a projection lens 67, and a prism body 30 is disposed between the two liquid-crystal panels 7a, 7b. The prism body 30 has, in its interior, reflecting planes 31, 32 orthogonal to each other. The liquid-crystal panel 7 corresponding to G is disposed at one side of the prism body 30 opposite to the projection lens 67.
The light source 35 is provided at an optical path inlet for the chassis 3, and a condenser lens 79 for concentrating light is disposed inwardly of the light source 35. Arranged on the optical path, inclined with respect thereto, are total reflection mirrors 75, 76, 77, 78 and dichroic mirrors 45, 46. The dichroic mirror 45 passes B while reflecting G and R. The dichroic mirror 46 reflects G while passing R.
B passing through the dichroic mirror 45 is reflected at the total reflection mirror 76, irradiates the liquid-crystal panel 7a corresponding to B, and is caused to impinge on the projection lens 67 by the reflecting plane 32 within the prism body 30. R is reflected from the total reflection mirrors 77, 78 and thereafter incident on the projection lens 67 upon reflecting at the plane 31 within the prism body 30. Thus, R, G, B to be incident on the projection lens 67 are combined by the prism body 30.
With the projector of FIG. 7, the optical path length of R is greater than those of G and B. It is therefore likely that R, emanating from the light source 35 and separated off, will diffuse before reaching the liquid-crystal panel 7b, failing to fully irradiate the liquid-crystal panel 7b.
To obviate this problem, convex lenses called relay lenses 80, 8, 81 are fixedly mounted on the chassis 3 between the dichroic mirror 46 and the liquid-crystal panel 7b to sufficiently irradiate the panel 7b corresponding to R. Of the three relay lenses 80, 8, 81, the middle relay lens 8 serves to make the emanating light into approximately parallel rays.
However, the above projector has the following problems.
The optical path is likely to deflect from the normal position owing to variations in the dimensions of optical components, such as lenses and mirrors, mounted on the chassis 3 and to errors involved in mounting. R, with an elongated optical path, is especially susceptible to this influence.
A minimum required range of irradiation is predetermined for liquid-crystal panels of the type mentioned. Suppose the minimum required range is a range A indicated in a solid line in FIG. 2. If the optical path is deflected from the normal position, the light incident on the liquid-crystal panel 7b will irradiate a range A1 of broken line, as displaced from the range A.
Consequently, R will not be accurately combined with the other rays G, B, producing color irregularities in the composed image, whereas the relay lenses 8, 80, 81, which are fixed to the chassis 3, are not adjustable for correcting the deflection of the optical path. For this reason, it has been conventional practice to set a wide range A2 including A and A1 and shown in FIG. 2 for irradiating the liquid-crystal panel 7b, and to set irradiation ranges A2 also for the other liquid-crystal panels 7, 7a.
Thus, color irregularities in the projected composite images are precluded by superposing R, G, B, with the ranges of irradiation made wider than should be set.
When the light source 35 has constant brightness, the brightness per unit area, i.e., illuminance, on the screen is inversely proportional to the square of the area of the irradiation range, as is well known, so that the illuminance is lower than is intended if a wider irradiation range is set.
Nevertheless, a higher illuminance is required of projectors of the type mentioned. The conventional projector, wherein the irradiation range is wider than is needed, fails to fulfill this requirement. In view of this point, the present applicant has conceived the idea of achieving a higher illuminance by adjusting the position of the relay lens to set a minimum required range of irradiation for the liquid-crystal panel 7b.